Abstract:
A species of slug (Arion subfuscus) produces a defensive gel it can chemically convert into a remarkably strong glue. Similar gel-based glues attach some snails firmly onto slippery rocks; tools are needed to pry them off. The tenacity of these glues on wet surfaces is difficult to match with artificial adhesives. Following up on their original research identifying the key characteristics controlling this transition from a water-based gel into a powerful yet flexible adhesive, researchers at Ithaca College have shed new light on the nature of the adhesive mechanism.

"The strength of the natural adhesive comes from the way long, rope-like polymers chemically tie together, or cross link, at certain points," said Andrew Smith, associate professor of biology. "In our previous studies we had shown that metals were essential to the formation of cross-links. This is unusual, as some combination of electrostatic and hydrophobic interactions are commonly responsible for the formation of cross-links in other gels."

Electrostatic interactions occur when a negatively charged group on one polymer is attracted to a positively charged group on another. Hydrophobic interactions take place when regions of a polymer don't interact with water, so they stick together to avoid contacting water.

"We used several approaches to break these interactions, and the treatments that normally disrupt them had no impact on the glue's mechanical integrity or ability to set," Smith said. "Our study conclusively showed that electrostatic and hydrophobic interactions do not play any detectable role. Removing metals alone caused the glue to fall apart. This was exciting and unexpected."

Removing the metals, however, didn't completely break down the gel. The researchers discovered that a specific protein was responsible for forming strong cross-links that were unaffected when the metals were removed after the glue set. But when metals were removed before the glue set, the cross-links didn't form."This is a very unusual material we're looking at," Smith said. "By discovering that metals are central to forming cross-links, we know there are several intriguing mechanisms that could hold the glue together."

For example, zinc, calcium and iron ions can bind very strongly to several molecules at the same time, thereby effectively joining them together. Iron and copper can also catalyze reactions that trigger strong cross-link formation.

"The significance of this is that we are much farther along the path to our goal of identifying how the glue works so that synthetic mimics can be made," Smith said.

The study, "Robust Cross-links in Molluscan Adhesive Gels: Testing for Contributions from Hydrophobic and Electrostatic Interactions," was published in "Comparative Biochemistry and Physiology-Part B: Biochemistry and Molecular Biology."